** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** EACH YEAR, SIX MILLION PREGNANT SOWS IN THE UNITED STATES ARE EXPOSED TO SUMMER HEAT STRESS, WHICH IS INCREASING IN FREQUENCY AND SEVERITY DUE TO CLIMATE CHANGE. IN UTERO HEAT STRESS (IUHS) OF MALE OFFSPRING IS ESPECIALLY DETRIMENTAL TO THE SWINE INDUSTRY BECAUSE BOARS ARE CRITICAL DRIVERS OF PIG PRODUCTIVITY AND IUHS IMPAIRS BOAR SEMEN QUALITY. THE OVERALL PROPOSAL GOAL IS TO BETTER UNDERSTAND HOW IUHS IMPAIRS BOAR REPRODUCTIVE PHYSIOLOGY WHILE SIMULTANEOUSLY TESTING A NOVEL IUHS MITIGATION STRATEGY -- GENOMIC SELECTION FOR HEAT TOLERANCE. IN OBJECTIVE 1, WE WILL DETERMINE HOW IUHS AFFECTS REPRODUCTIVE ENDOCRINOLOGY OF THE BOAR. OBJECTIVE 2 WILL ASSESS HOW IUHS IMPACTS SPERM PRODUCTION, SPERM HEALTH, AND FERTILITY. IN OBJECTIVE 3 WE WILL DETERMINE HOW IUHS IMPAIRS CELLULAR AND MOLECULAR MECHANISMS CONTROLLING BOAR TESTIS FUNCTION. WE HYPOTHESIZE THAT IUHS WILL IMPAIR TESTIS DEVELOPMENT AND ENDOCRINE FUNCTION, WHICH WILL DECREASE SPERM PRODUCTION AND FERTILITY OF BOARS. THE PROPOSED RESEARCH IS SIGNIFICANT BECAUSE IMPLEMENTATION OF THIS NEW KNOWLEDGE IS EXPECTED TO LEAD TO NOVEL METHODS (E.G., NEW GENETIC TESTS) TO ENHANCE TESTICULAR FUNCTION OF BOARS EVEN IN THE FACE OF CLIMATE CHANGE. ULTIMATELY, THESE OUTCOMES WILL INCREASE BOAR FERTILITY, SWINE REPRODUCTIVE EFFICIENCY, AND THE SUSTAINABILITY OF U.S. PORK PRODUCTION.